1. Field of the Invention
The invention relates to an arrangement for holding the top sheet of a stack of sheets ejected by an office machine and deposited on the stack.
2. Related Art
The sheets ejected by office machines such as printers, copiers, etc. are generally collected in a stack. The sheet ejector of an office machine ejects the sheets either individually or, for example, as already collected print jobs. If the ejected sheets come into contact with the top sheet of an already collected stack, there is a risk that the top sheet of the stack may be displaced by the ejected sheet, thus upsetting the alignment of the sheet stack. The pull of the top sheet on the stack by the ejected sheets depends on various factors such as the surface condition of the sheets, electrostatic charges, weight of the sheets, number and size of the sheets, and the air humidity, for example.
To avoid the displacement of the top sheet on the stack by the following ejected sheets, it is known to place a holding element on the top sheet of a stack while the next sheet is ejected. The holding element holds the top sheet of the stack while the next sheet is deposited on the stack. The holding element is then moved away from the stack so as to not prevent the stacking and alignment of the next ejected sheet.
One problem with this arrangement is placing the holding element on the top sheet of the stack with a precise placement force. A minimum of placement force is required to dependably hold the top sheet, but excessive placement force can lead to pressure marks on the sheet. With the known devices, it is difficult to maintain a precise placement force of the holding element, because the position of the upper edge of the stack is undefined. The position of the upper edge of the stack may be influenced, for example, by air pillows between the stacked sheets. Especially, there is frequently a difference in the height of the upper edge of the stack across the width of the stack resting on the alignment stop, which is particularly pronounced when several already collected and stapled sheets are deposited and stacked.